1. Field of the Invention
The present invention relates to an optical measurement device wherein an electric current is measured by applying the principle of the Faraday Effect, and in particular it relates to the construction of the optical system mounting box.
2. Discussion of the Related Art
An optical current transformer for GIS used in a power GIS system is described with reference to FIG. 1 as one example of a prior art optical measurement device.
A conductor 8 is arranged in a GIS tank 10 which contains, scaled within it, insulating gas at a pressure of a few atmospheres. This is covered by a sensor 2 consisting of quartz or lead glass etc. processed and assembled in such a way as to surround it. Light emitted from a light source is directed, through an optical communication fibre 71, into an optical system mounting box 31, where it is converted into prescribed linearly polarized light and reaches sensor 2 after passing through pressure-scaling window 11 which is arranged so as to maintain the gas pressure in GIS tank 10. Linearly polarized light passing through the vicinity of conductor 8 by repeated reflection in sensor 2 is again directed into optical system mounting box 31 by passing through pressure-sealing window 11. By the principle of the Faraday Effect, the plane of polarization is rotated in response to the current flowing in conductor 8 in proportion to the intensity of the magnetic field generated in its periphery, with the result that the linearly polarized light that re-enters optical mounting box 31 is rotated in accordance with the amount of the current of conductor 8. This is then subjected to vector resolution into mutually orthogonal X and Y components in optical mounting box 31 and exits by respective X-component receiving fibre 72 and Y-component receiving fibre 73. After this, it reaches a photodetector, not shown, where it is converted into an electrical signal, and the current value is then calculated by a current processing circuit.
Describing the construction of optical system mounting box 31 in more detail, incident light received by fibre 71 is converted into a parallel spatial beam by a lens 33, converted to linearly polarized light by a polarizer 34 comprising a calcite Glan-Thompson prism or the like, and output at the pressure sealing window 11. In contrast, the linearly polarized light that re-enters optical system mounting box 31 after rotation enters an analyzer 35 comprising a calcite Wollaston prism and/or polarizing beam splitter or the like and is separated into mutually orthogonal directions. This X component beam and Y component beam are collected by respective lenses 33 and are then output onto the X component receiving fibre 72 and Y component receiving fibre 73. Optical component mounting elements 32 are used to fix these optical components in position with their axes aligned.
However, such a construction is subject to the following problems.
The distance through which the optical flux is propagated in the gas is too long to maintain high accuracy. For example, if the optical flux is input to the optical fibre from a given optical component, if the distance between these two is 5 mm, and if the core of the optical fibre is a few microns in diameter, an accuracy of about 10,000 times is required. Furthermore, since the refractive index of the gas fluctuates due to changes in ambient temperature, the direction of the beam fluctuates, resulting in fluctuations in the amount of light passing between the optical components.
To maintain high accuracy with such a construction, the construction between the various structural components in optical system mounting box 31 is too complicated. More specifically, thermal strain produced by changes in ambient temperature results in displacement of the optic axes of the components with respect to each other and the construction is unstable in regard to vibration and shock.
Because of the above problems, the overall size of the optical current transformer becomes large. This militates against the demand for miniaturization of the GIS as a whole.
This invention is made in order to solve the aforementioned problems. Its object is to obtain an optical measurement device that can be made of small size and light weight without impairing its accuracy under ambient temperature variations.